Straight 70% or 90% isopropyl rubbing alcohol - touch insects with a soaked cotton swab, or for larger infestations, spray the entire affected plant, being sure to thoroughly wet all surfaces.  Repeat every 3 days for about 2 weeks.  (I have never experienced any problems with buds, flowers, or any part of the plant when using an alcohol spray.)

Garlic/pepper spray - liquefy 2 bulbs of garlic and 2 hot peppers in a blender 1/2 to 2/3 full of water. Strain to remove the solids and add enough water to the garlic/pepper juice to make 1 gallon of concentrate. Use 1/4 cup of concentrate per gallon of spray. To make garlic tea, simply omit the pepper and add another bulb of garlic. Add two tablespoons of blackstrap molasses for more control. 

Mix 1 teaspoon (5 ml) each cooking oil and liquid dishwashing soap or detergent in a quart (liter) of water.  Concentrations are not critical - some recommend a tablespoon (15 ml) of oil be used.  Shake well, and spray the plant, being sure to thoroughly wet all surfaces.  Repeat every 3 days for about 2 weeks.  (The soap breaks down the waterproof, waxy coating on the insects' shells, and the oil will smother them.)

One cup (250 ml) each Formula 409 or Fantastik household cleaner/degreaser and isopropyl rubbing alcohol and 2 cups water, making a quart or liter of spray.  Use the same as the formulas above.  (I've not tried this one.)

Orange Plus, a household cleaner made from byproducts of orange juice production, can be sprayed directly on plants to eliminate insects.  (I suspect other concoctions made using citrus oils may also be effective, but have no knowledge or experience concerning them.)
 

in a 1 quart spray bottle mix 1 tsp Murphy's Oil Soap, 1 tsp Sunspray (or cooking oil), 1tbs lemon juice, and fill the rest with 1/2 water and 1/2 alcohol.

Melt about 1/4 pound of candle wax, then slowly stir in about 1/4 cup sugar and 4 ounces boric acid powder (available at your local drug store).  When thoroughly mixed, pour into a pan, creating a 1/4" to 1/2" thick slab.  Cut or break into chunks, and distribute around the greenhouse.  If you decide to try this indoors, be sure to keep them away from children and animals.

A surprising  - and no doubt smelly - ant repellant is fermented cow manure tea.  Put about a gallon volume of fresh manure in a 5-gallon bucket and top it up with water.  Cover and let stand for a couple of weeks or more until it's fully "cured."  Spray around the greenhouse and under the benches.  (CJ Maciejeski claims this keeps fire ants away down in Houston, and if it works for them...)

Here's another volunteered from the world of cyberspace:  raw grits or corn meal!  Sprinkle them in an area where ants are attracted, and they will gobble them up.  Later they will swell in their bellies. (Need I say more?)
 

Another contributor verified that the grits or corn meal works, but only for larger ants.  For the small ones, try sprinkling some bath powder around.

Along those same lines, a 50/50 mix of powdered (confectioner's) sugar and baking soda is supposed to be effective against ants.
 

OK, another one volunteered by a reader - aspartame!  You know...Nutrasweet, the sugar substitute?  Apparently it is attractive to them, is taken back to the colony, and kills the lot.  This one is even supposed to be effective against fire ants.  You may find it necessary to dampen the powder or granules to make it attractive to the critters.

Place small, open containers of eucalyptus oil in the growing area.  The vapors will discourage critters from approaching.  (Yeah, it's going to smell like a Hall's Cough Drop factory, but it's good for opening your sinuses, too - a la Vick's Vapo-Rub.)  Eucalyptus oil has also been shown to be an effective fungicide, but I know that a dispersion in water and alcohol, when sprayed directly on the plant, will damage flower buds, so I can't recommend that use.

Apply normal, household cinnamon powder directly to the affected part of the plant by dusting heavily.  This has proven to be a good way to control slime mold and mushrooms in the mulch in my outdoor flower beds, too!

Mix cinnamon powder with sufficient casein-based glue (Elmer's) to make a thick, brown paste.  Apply to the wound and let dry.  The Elmer's Glue is water soluble, but resists washing-off quite well.  This is the preference for mounted plants that get watered or misted frequently.
 

An alternative to the Elmer's Glue, but just as waterproof and long-lasting is made by mixing cinnamon powder and cooking oil to form a thick paste. (Thanks to John Kawamoto!)

Put 2 tablespoons (30 ml) of cinnamon powder in a pint (500 ml) of isopropyl rubbing alcohol.  Shake well and let stand overnight.  Filter the solution to remove the sediment (coffee filters work well), and use the brown liquid as a spray.  (While it's not a big problem for most orchid growers, I've heard that this is good for powdery mildew, as well.)

Put the cinnamon powder in hot water.  Shake well and let stand for several days.  Filter and use as above. (Some feel that the alcohol can be too desiccating when used on seedlings.)

Mix approximately 1/3 cup milk into a quart of water, and spray.  I have not tested this one, but even if it doesn't work, you end up with shiny leaves!

Mix 3 tablespoons cooking oil, 1 tablespoon liquid dishwashing detergent, and 1 tablespoon baking soda in a gallon of water; spray at three-day intervals for powdery mildew.

Trap:  Put a plate or plant saucer full of fresh beer on the floor; snails and slugs will be attracted to it, fall in and drown.  (Stale beer, has apparently been shown in university studies, to be a repellant, not attractant!

Spray:  Mix one cup household (non-sudsy) ammonia with water, and spray directly on the critters.
 

Spray:  Don't throw away the left-over coffee!  Mix it 50/50 with water (some say use it straight), and spray.  I add about 2 ounces per gallon rubbing alcohol to keep mold from growing on the liquid surface when the stuff is stored.  Seems to work great on Bush Snails.

Mechanical:  Spread a layer of Diatomaceous Earth on the benches, around your plants.  The material - the skeletons of microscopic sea creatures (diatoms) - is almost pure silicon dioxide, and has very sharp points and edges that discourage the passage of the creatures.  The material sold as a microfiltration medium for aquariums probably won't do much.  If you can find horticultural grade material, it is coarser and has much sharper edges that make a great barrier.
 

Mechanical II:  Staple wet/dry sandpaper to the legs of your benches, grit side out.

Mechanical / Electrochemical:  Tightly wrap the legs of your bench with a 2" (5cm) wide strip of copper foil, being sure to apply it tightly enough to avoid gaps.  The copper is supposed to create some sort of uncomfortable electrochemical effect when in contact with the "slime" secreted by the critters.  I suppose it's like biting on a piece of foil if you have metal fillings in your teeth!

Chemical:  Apply a thick layer of the product Tree Tanglefoot around the legs of the bench.  Available a most good garden centers, it is sold as a bird repellent for ornamental trees, it contains a castor-bean extract that repels slugs and snails.

If you just want to lure the critters away from your plants so you can dispose of them, try putting slices of raw potato near the potted plants on the bench.  The slugs and snails can be found on the underside in the morning. (Thanks to Janet Price.)

Place one cup of whole ground corn meal in an old sock or panty hose leg and tie a knot in it to contain the meal.  Place in the water tank for the cooling system.  The algae will dissolve in a few days.  Note:  I have no idea if this methodology can be used for algae growing in pots.)

Spray full-strength household vinegar on weeds, repeat daily as needed.  This is NOT to be used for weeds growing in the pots of your other plants, but is good in driveways, walkways, the greenhouse floor, patios, etc.

Add one ounce of household chlorine bleach to a gallon of water and spray on utensils, benches, even your plants to control a wide variety of pathogens.  It even helps control algae.